Planets, in particular habitable planets, are so common in works of science fiction that there’s a tendency to assume that they’d be common in the real Universe. There is little hard data to support that notion–not yet anyway. Just 15 years ago, the only planets astronomers knew where the nine that orbited one star: Sol. (I’m not attempting to promote Pluto-back-to-full-fledged-planethood, but it was considered a planet back then, hence the inclusion.) We have now identified over 490 planets (and counting) orbiting other stars. So although stars with planets seem to be fairly ubiquitous, perhaps even the rule rather than the exception, that still raises the question of the abundance of habitable planets.

Until recently the detection methods astronomers used for finding extrasolar planets has had a distinct bias–the planets we’ve found tend to be large, Jupiter-like, and close to their parent stars. Now the Kepler spacecraft has just begun its search for extrasolar Earths and, in a very short time, has already found over 700 candidate stars that could have Earth-sized planets. As followup studies examine these candidate stars further, is it only a matter of time until another “Earth” is detected? Certainly, but we may have to sift through a lot of near-misses first.

New Scientist has a interesting article on whether or not there is a Moore’s Law for Science, using the extrasolar planet hunt as backdrop for examining whether or not previous rates of scientific discovery can be used as predictors of future performance. The article says:

Their calculations suggest there is a 50 per cent chance that the first habitable exo-Earth will be found by May 2011, a 75 per cent chance it will be found by 2020, and a 95 per cent chance it will be found by 2264.

Is there a 75% chance we’ll find an Earth-sized planet by 2020? Almost certainly, given the performance of the Kepler spacecraft and the fact that astronomers have found a planet only 1.5 times larger already. A habitable exo-Earth? Not so fast.

Enter CO2. Earth-sized planets that are situated in the habitable zones, or Goldilocks zones, of their parent stars are too small and too warm to hold onto the two most common gases: hydrogen and helium. Terrestrial planet atmospheres, at least the ones with which we are familiar, are formed initially from the volatile compounds commonly found in, and delivered by, ices from comets: in particular water (H2O), ammonia (NH3), methane (CH4), and carbon dioxide (CO2). At a molecular level, CO2 is, by far, the most massive of those four compounds.

So, like dry ice (also CO2) fog at a Halloween party, CO2 sinks to the bottom of a planet’s atmosphere, displacing other gases that can eventually escape into space. When we look at our neighbors, both Venus and Mars have atmospheres composed mostly of CO2. Venus is so hot that the molecules of most gases easily reach escape velocity. (Carbon dioxide is a greenhouse gas that traps the radiant heat from the sun efficiently, driving the temperature of Venus to approximately 860 degrees Fahrenheit planetwide.) Although Mars is far colder, it has only 37% of Earth’s gravity; it’s so small that the molecules of most gases escape, just as on bigger, hotter Venus.

Earth, too, likely had an atmosphere composed chiefly of CO2 in its youth, and if it still had that atmosphere, it would be too hot for life. Earth was “lucky”, though: in its infancy Earth was struck by a Mars-sized object and stripped of its Venus-like atmosphere. What are the odds that events like this are common throughout the galaxy?

So we’re not necessarily on the brink of finding Caprica, Minbar, Risa or, luckily, Skaro. Even though our detection methods are likely to turn up numerous Earth-sized planets in the very near future, they’re unlikley to be Earth-like. Yes, it’s only a matter of time until we find the first exo-Earth, but given the relative abundances and properties of the most common gases that form terrestrial planet atmospheres, we may run across a lot of extrasolar Venuses first.

Comments (11)

I was under the impression that the Earth has approximately the same amount of carbon as Venus, the main difference being that on Earth, plate tectonics keeps most of the carbon locked up in the mantle and crust.

Does CO2 sink to the bottom of an atmosphere?
I don’t think so. Once gasses – even of widely differing molecular weights- are mixed, they stay mixed.
Now – if you’re talking about which gasses achieve escape velocity from the rarified upper atmospheres, that’s fine. But gasses in the lower, more dense part of the atmosphere will not stratify.
The other exception would be a planet that gets cold enough to liquify gasses – they’ll drop out of their gasseous states at different temperatures. Having become unmixed, they could remain stratified and relatively unmixed after that region warms up enough to re-vaporize the liquids.

1. Gases in atmospheres of planets similar to Earth are well mixed. The mass difference is too small to offset the mixing from convection. ChH’s idea that gases might stay unmixed requires a planet without convection, which can only happen without a greenhouse effect, thus without CO2 in the mix.

What can happen is that H2O at high altitudes is dissociated into hydrogen and oxygen so the hydrogen is lost. On Earth the tropopause is cool enough that almost all water freezes out before it can get that high leading to a very dry stratosphere. The UV needed to dissociate water molecules doesn’t reach deep enough into our atmosphere to find much water to attack. If not for this, Earth might have lost most of its water by now.

The idea that earth has lost its carbon dioxide is spurious. It’s locked up in the massive beds of calcium carbonate (limestone & marble) or converted to free oxygen and coal beds all over the world. On Venus it has been baked out just as we bake it out when we make cement from limestone. There are no oceans full of life on Venus to bind it again as limestone, coal, and oil.

First, on the atmospheres and their mixing, the comments have mentioned what is observed, well mixed atmospheres. It should also be noted that if this didn’t happen, there would be little greenhouse effect as it happens (mostly) throughout the entire mixed troposphere.

Second, on the metrics, the habitability prediction is using observed gravity and surface temperature, so is safe from atmospheric compositions which are already folded in.

But even so, if you start to include atmospheric composition you will differentiate between inhabited habitable planets (nitrogen/oxygen atmosphere in Earth analogs, nitrogen/methane in putatively inhabited Titan analogs, no atmosphere in putatively inhabited Europa analogs) and uninhabited habitable planets. That is something else entirely.

Earth, too, likely had an atmosphere composed chiefly of CO2 in its youth, and if it still had that atmosphere, it would be too hot for life. Earth was “lucky”, though: in its infancy Earth was struck by a Mars-sized object and stripped of its Venus-like atmosphere. What are the odds that events like this are common throughout the galaxy?

The odds with your model is 1/3, extrapolating from observations of our system.

All four terrestrials were hit with massive last impactors scaled to their final size. (Moon sized for Mars, potentially the one creating both the North cap low region and the Moon analog Phobos & perhaps Deimos impact ejecta assemblages. There’s an interesting update on the later prediction circulating right now.) Mercury atmosphere was lost anyway, too close to the Sun, so we don’t know either way.

Venus last impactor was likely larger than ours since it retrograded Venus. The difference is that the retrograde motion would destabilize any Venus moon impact ejecta assemblages by tidal effects, to crash back on Venus. (See for example Wikipedia on Venus for references.)

But that isn’t what is thought happened. The main model today seem to be that early Earth retained a massive CO2/H2O atmosphere after the Theia impact. In collision models Theia atmosphere can contribute to Earth’s volatile supply, at that, explaining why Earth/Moon refractories are well mixed but Earth got plenty volatiles. And certainly our carbon supply supports that hypothesis. (See astrobiology textbooks.)

The difference between Earth and Venus then comes down to plate tectonics that locked away our carbon. Earth retained enough water after the putative early water hydrolysis/hydrodynamic hydrogen loss stage to form seas making the crust malleable to plate tectonics by modifying viscosity such as by helping granitification along. Venus did not, probably because it was closer to the early sun massive CMEs and lost too much hydrogen.

What this means is that the habitable zone for migrating planets is initially smaller (outwards Venus) than later (inwards Venus).

To conclude, Earth locked away substantial carbon quickly since the early crust formation was quick. (Some papers now say less than ~ 30 My, because of the new find of material from an early differentiated crust reservoir.) And since its initial turn around was an order of magnitude faster. (~ 5 times larger internal heat flow, and no large plates somewhat lidding.)

We know from both fossil water isotope analysis and and an ingenious new chert isotope analysis method that Earth seas was less than 40 degC @ ~ 3 Ga. Certainly the stromatolite cyanobacteria assemblages @ ~ 3.5 Ga was living in less than 73 degC, the upper limit for aerobic photosynthesis such as they use. Finally, isotope analysis of diamonds locked in the oldest found zirconium crystals put the seas as below 100 degC @ ~ 4.2 Ga, because they saw a substantial liquid water supply. At that time the solar radiant flux was ~ 70 % today’s level, mind.

In sum these upper temperature limits tests the carbon dioxide lock in prediction well, seeing the initial available carbon supply and the resulting one predicts the process in the first place.

You know what I hate about all these kinds of articles? People just dont really do the math. I mean, seriously, what are the chances of intelligent life happening in a universe as vast as ours more than once? I would say probably 100% even though we may never find intelligent life on another planet, I just wish people would get it through there head that its surely out there. Even if life is not common, if life were the exception rather than the rule, there would still be at least 1 other planet out there with intelligent life, and considering the size of galaxies, I am pretty sure that there are thousands of habitable worlds in our galaxy alone, if not millions. Just because we cant see them or talk to them does not mean that there not there!